Suspicious behavior on T-Mobile US Inc.’s network devices tipped off the company to a breach that was potentially part of a sprawling cyber-espionage campaign that has raised urgent questions about the exposure of a critical sector of the economy.Jeff Simon, T-Mobile’s chief security officer, said in an interview with Bloomberg News that while the behavior wasn’t “inherently malicious,” it was unusual enough to draw the attention of the company’s network engineers. In recent weeks, the engineers...